Conventionally, molding of light metal alloys has been carried out using a die casting method exemplified by a hot chamber method and a cold chamber method. In particular, magnesium alloy molding is also carried out using thixotropic molding as well as the above-described die casting methods.
Die casting methods involve supplying molten light metal material that has been melted in a furnace beforehand to the inside of an injection cylinder of an injection unit, and injecting the molten metal into a mold using a plunger. With this type of method, high temperature molten metal is supplied stably to the injection cylinder. In particular, with the hot chamber method, since the injection cylinder is arranged inside the furnace, high temperature molten metal is supplied to the mold in a fast cycle time. Also, with the cold chamber method, since the injection cylinder is arranged separately from the furnace, it is easy to carry out maintenance of the injection unit. On the other hand, with thixotropic molding, small pellet-shaped magnesium material is melted into a semi-molten state by shearing heat due to rotation of a screw and heat from a heating system, and then injected. The injection device for this molding is constituted by one of two types of units, as described in the following. One type of unit is the unit disclosed, for example, in Japanese Patent No. 3258617 provided with a melting unit for melting light metal material in a semi-molten state using a screw inside an extrusion cylinder, and an injection unit for injecting molten metal supplied from the melting unit to the inside of an injection cylinder, with connection between the extrusion cylinder and the injection cylinder being made using a connecting member. Another type of unit is a unit having basically the same structure as an in-line screw type injection machine, for carrying out melting and injection with a single cylinder having an in-line screw built-in. The latter structure is fairly general, and so disclosure of prior art documents, such as patent documents, will be omitted. In any event, the injection molding machine using these thixotropic molding methods has the advantage that there is no need to provide a large capacity furnace required for a die casting method.
However, with the above-described molding methods, there is a problem with the following improvements. First of all, with the die casting method, since a large capacity furnace is used the unit accompanies increase in scale, and since a lot of molten metal is kept at a high temperature the unit results in increased running costs. Also, because it takes a long time to raise the temperature of the furnace, maintenance of the furnace takes at least a day. In addition, particularly in the case of using magnesium alloy, it is extremely easy for magnesium to be oxidized and to catch fire, which means that oxidization prevention measures for the molten metal and adequate fire prevention measures are required. It is therefore necessary to inject a lot of non-burning flux or inert gas into the furnace. On top of this, since sludge having a main component of magnesium oxide is generated even if such counter measures are adopted, it is necessary to carry out sludge clean-up operations regularly. This sludge also causes wear. On the other hand, with the thixotropic molding, melting of the pellet-shaped material is carried out by rotating a screw, which means that it is not always easy to consistently melt the material to a desired semi-molten state. In particular, with an in-line screw type injection molding machine, since metering is carried out while causing the screw to retreat, skill is required in adjusting molding conditions. It is also easy for a screw and check ring to become worn. Also, since the molding material is a pellet-shaped material causing an increase in the surface area, it is easy for oxidation to occur. As such, it is necessary to consider the handling of the material.
In view of the above drawbacks, alternative injection devices have been proposed. One example is the injection device disclosed in Japanese Patent Laid-Open No. Hei. 05-212531. This injection device is an injection cylinder which comprises a metal mold side (front side) high temperature cylinder section, a rear side low temperature cylinder section, and a heat insulating cylinder section between them. With this injection device, molding material formed into cylindrical bars in advance is fitted into the injection cylinder and melted inside the high temperature cylinder section, and the molten metal is extruded and injected using not-yet melted molding material. Since the molding material itself injects without using a conventional plunger, in the present specification the molding material with this molding method will be called a self-consumption plunger. Since this type of injection device is not provided with a furnace, the volume of molten metal is reduced as a result of simplifying the vicinity of the injection device, which means that effective melting is likely to be made possible. Also, since this type of injection device is not provided with a plunger, it is possible to reduce wear of the injection cylinder and to carry out maintenance in a short time.
Further, similar techniques are also subject of patent applications by the same applicant (for example, Japanese Patent Laid-Open No. Hei. 05-238765 and Japanese Patent Laid-Open No. Hei. 05-254858). These documents disclose injection devices for glass molding, but because they use the self consumption plunger they are similar techniques. Specifically, Japanese Patent Laid-Open No. Hei. 05-238765 discloses a seizing-up prevention technique, in which pluralities of grooves or spiral grooves are formed in advance in a cylinder side, and molding material is cooled by circulation of a cooling medium in these grooves. Also, Japanese Patent Laid-Open No. Hei. 05-254858 discloses the seizing up prevention technique, where pluralities of grooves or spiral grooves are formed in a molding material (self consumption plunger) side; and are absorbing diameter expansion and deformation of softened molding material. Since glass is supplied in a high viscosity softened state in a comparatively wide temperature range and molten metal is not directly embedded in the grooves, the grooves can be used effectively in preventing seizing up of the glass material.
However, Japanese Patent Laid-Open No. Hei. 05-212531 described above does not disclose a technique that is practicable with respect to the length of molding material, structure of a injection device and a molding operation itself. For example, Japanese Patent Laid-Open No. Hei. 05-212531 discloses nothing about solving that at the time of injection, low viscosity molten metal flows backward at high pressure in a gap between the injection cylinder and the self consumption plunger, and as a result is solidified, rendering movement of the plunger impossible. This phenomenon often arises when the injection device is injecting light metal material and is more pronounced when carrying out injection at high speed and high pressure. This is because solidified matter of the molten metal is often destroyed, re-formed, and then grows to be the stronger solidified matter at time of injection operation.
No method for solving this type of phenomenon is disclosed in either of the above disclosed patent Japanese Patent Laid-Open No. Hei. 05-238765 or Japanese Patent Laid-Open No. Hei. 05-254858. The reason for this is that in the case of using these molding devices in molding of light metal material, since molten metal quickly infiltrates into the grooves and is solidified over a wide range, the grooves do not function as cooling grooves or as deformation absorption grooves. More specifically, this is because the molten metal solidifies accompanying immediate entry into the grooves since light metal melts or solidifies quickly due to the small specific heat and latent heat and high thermal conductivity inherent to light metal, since the temperature range of material exhibiting a softened state is narrower than that of glass, and since molten metal exhibits extremely low-viscosity fluidity. As a result, the above-described operational effect of the grooves is not demonstrated in cases such as glass molding due to filling of the solidified matter. Since these patent documents disclose techniques for preventing seizing up of glass material in a glass molding injection device, naturally they are relevant.
The object of the present invention is to provide an injection device capable of efficiently supplying light metal material to a melting unit, and also capable of more reliably, efficiently and stably supplying molten metal to a plunger injection device, by proposing a characteristic light metal material supply method and an injection device including a characteristic melting unit for effectively handling this supply method. A further object of the present invention is to provide a melting device and a plunger injection device capable of reducing wear and suppressing backward flow of molten metal from a melting cylinder during metering or from an injection cylinder during injection. The other operational effects achieved using such a structure will be described together with a description of embodiments.